Without getting into exponential functions this can be done rather easily.
For every half-life, one half of an isotope decays.
26.3 / 5.26 = 5
26.3 years is exactly 5 half-lives.
After EACH half-life we have one half of what we previously had.
So after one half-life we have 50%
2 half-lives 25%
3 half-lives 12.5%
4 half-lives 6.25%
5 half-lives 3.125%
So, how much is left of the 200 gram sample?
200 * .03125 = 6.25 grams
